This invention relates to an aspect ratio discrimination apparatus and an image display apparatus wherein the image display mode can be changed over automatically when, for example, a horizontally elongated screen having an aspect ratio of 16:9 tries to exhibit an image of an aspect ratio different from the aspect ratio of the screen.
Recently, image display apparatus such as television receivers which include a cathode ray tube (hereinafter referred to simply as CRT) of an aspect ratio of, for example, 16:9 and can display a wide screen have spread widely. If, for example, a movie is exhibited on such a television receiver of the wide type as just mentioned, an appealing image as may be enjoyed in a movie theater can be obtained.
When trying to exhibit a normal image having an aspect ratio of 4:3 on a CRT having another aspect ratio of 16:9, since the size of the screen of the CRT and the size of the image are different from each other, various picture modes are provided in order to reduce an unfamiliar feeling arising from the difference in size.
For example, FIG. 3A illustrates a normal mode in which an image P1 of the aspect ratio of 4:3 is displayed on a screen D. In this instance, black bands B for filling up the opposite horizontal end portions of the screen D are displayed so that the image P1 of the aspect ratio of 4:3 is displayed at the center of the screen D in the horizontal direction.
Then, if the amplification level of the horizontal scanning signal for the image P1 of FIG. 3A is raised to expand the image P1 in the leftward and rightward directions so that it may fit the screen D, then a just mode is obtained wherein the image P1 is displayed as a horizontally elongated image P2 of the aspect ratio of 16:9 as shown in FIG. 3B.
Recently, the use of an image obtained by adding black bands B to the upper and lower ends of an image P1 produced with the aspect ratio of 16:9, for example, in a movie or in a relay broadcast of a sports program and supplied as an image of the aspect ratio of 4:3 is increasing.
In this instance, an image is displayed on the screen D, as shown in FIG. 3C, as a letter box image wherein the black bands B are displayed above and below an image P3 of a normal mode. Since the actual image displayed here is displayed with the aspect ratio of 16:9, the amplification levels of the horizontal and vertical scanning signals may be changed to expand the image in the horizontal and vertical directions so that it can be displayed in a zoom mode in which an image P4 just fits the screen D as shown in FIG. 3D.
Also a yet further mode is known wherein, for example, as shown in FIG. 3E, the length of a screen D in the horizontal direction and the length of an image P in the horizontal direction are set equal to each other and an image P5 is exhibited with an upper portion and/or a lower portion thereof trimmed as indicated by broken lines. In this instance, if the image P5 is scrolled upwardly or downwardly, the upper portion or the lower portion of the overscanned image can be observed.
Among such various picture modes as described above, the effective picture mode used by an image display apparatus can be changed over either by manual operation of a user or automatically by discriminating, for example, whether an inputted image has the aspect ratio of 4:3 as seen in FIG. 3A or the aspect ratio of 16:9 with black bands provided above and below the image as seen in FIG. 3C.
In order to perform automatic mode change-over, for example, as schematically shown in FIG. 4, brightness levels of an upper portion and a lower portion of an inputted image are compared with a reference voltage Vref, and when the brightness levels are lower than the reference voltage Vref, the inputted image is determined to be such an image P3 of the aspect ratio of 16:9 as shown in FIG. 3C with black bands B provided above and below the same. Thus, the picture mode is changed over, for example, to the zoom mode illustrated in FIG. 3D.
On the other hand, when the result of comparison reveals that the brightness levels are equal to or higher than the reference voltage Vref, it is determined that an image is present at an upper portion and a lower portion of the screen. Thus, for example, if such an image P1 of the aspect ratio of 4:3 as shown in FIG. 3A is inputted, then the picture mode will be changed over to the normal mode of FIG. 3A or to the just mode shown in FIG. 3B.
Accordingly, for example, when an image P6 as shown in FIG. 5A is detected as an image of the aspect ratio of 4:3, control to display the image P6 in the normal mode or the just mode is performed. On the other hand, when an image P7 as shown in FIG. 5B is detected as an image having the aspect ratio of 16:9, control to display the image P7 in the form of a letter box image or in the zoom mode is performed.
However, when, for example, a video tape is used as an image source and a movie or the like wherein an image is provided with black portions above and below the same having the aspect ratio of 16:9 is reproduced, if noise or the like is superimposed on the black bands as a result of deterioration of the video tape or by a like cause, then the black level is raised by it so that it is discriminated as if there is some image. Accordingly, a malfunction results since, although an image of the aspect ratio of 16:9 is exhibited, control for exhibiting an image of the other aspect ratio of 4:3 is performed. Further, particularly where a U/V (UHF/VHF) (ground wave) broadcast is received, there is a problem in that a similar error occurs with regard to an image with which noise is superimposed by a weak electric field or the like.
Further, for example, depending upon mediums such as a television broadcast, a video tape and a laser disk, where the black band portions are not very black but have, for example, a black color near to gray or where noise is superimposed with the black bands B in a signal recorded on a video tape or the like, the black level sometimes exhibits a raised level. Also for those cases, it is a possible countermeasure to set the reference voltage Vref high in order to allow detection of a horizontally elongated image with a high degree of accuracy.
However, when the reference voltage Vref is set high, if, for example, as shown in FIG. 5(c), there is an image P8 of the aspect ratio of 4:3 wherein a car is running with the headlights thereof lit in a scene of night in a movie or a drama in which the screen is generally dark, then dark portions at the upper portion and the lower portion of the screen are sometimes determined as black bands. Consequently, although an image of the aspect ratio of 4:3 is exhibited, a malfunction results since control to exhibit an image of the aspect ratio of 16:9 is performed.